AS/A-level Chemistry - Ionisation Energy 

Ionisation is all about removing electrons from atoms to make positive ions.

The definitions of ionisation energies are always asked in AS/A-level Chemistry, make sure you understand them well! 👨‍🏫

First Ionisation Energy

The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions:

M(g)➔M⁺(g)+e^-

Second Ionisation Energy

The second ionisation energy refers to the removal of the next mole of electrons from the mole of gaseous 1+ ions:

M⁺(g)➔M²⁺(g)+e^-

The third ionisation energy refers to M²⁺(g)➔M³⁺(g)+e^-, and so on.

Do you remember the content of AS/A-level Chemistry regarding Periodic trends in the size of atoms and effective nuclear charge? 🧐 As they are factors that influence the first ionisation energy ❗️❗️

Factors influencing the first ionisation energy

• Nuclear charge - a large nuclear charge means the outer electron is difficult to remove, so the ionisation energy is large.
• Atomic radius - in larger atoms, the outer electron is farther away from and less strongly attracted by the nucleus, so it is easier to remove and the ionisation energy is lower.
• Electron shielding - if more inner electron shells shield the nuclear charge from the outer electron, it will be easier to remove, and the ionisation energy is low.

Ionisation energies across a period

• In general ionisation energies increase across the period. 
• This is caused by the increasing nuclear charge attracting the electrons more strongly and therefore making it difficult to remove an electron. The outer electrons of these elements are in the same shell, so atomic radius and electron shielding remain similar across the period.

Ionisation energies down a group

• The first ioniastion energy decreases down the group.
• This is because for each successive element, the distance from the nucleus and electron shielding from inner electrons both increase, and these out weight the effect of increasing nuclear charge.

How to predict electron configurations from ionisation energies

The values of successive ionisation energies can tell us the group an element is from. Successive ionisation energies (IE) simply mean the first, second, third, etc. IE for one particular element.

We can see that the pattern of successive IEs for:

• all group 1 elements is 1 low IE.
• all group 2 elements is 2 low IEs.
• all group 3 elements is 3 low IEs.

Then there is a large jumps in the IE as we enter another shell - the next electron is nearer to the nucleus and has fewer electrons shielding it from the nuclear charge.

Reference:

• 2.3.1: Ionization energy - Chemistry LibreTexts

CLICK HERE TO LEARN MORE ABOUT OUR A-LEVEL CHEMISTRY COURSES

Drafted by Cherry (Chemistry)